The invention relates to monitoring devices and more particularly, to methods and devices for monitoring the amount of fluid being applied to items in a fluid application unit. Specifically, said methods and devices are devised for use in a can production apparatus, for monitoring the amount of liquid lacquer applied to the outside of cans.
In an apparatus for production of cans, such as beverage cans of aluminum or steel material, a printer unit provides a graphical print on the circumferential surface area of cylindrical cans fed thereto. In some cases, the printer unit is preceded by a lacquer application unit, in which the cans are first provided with a so-called base-coat and then dried in an oven. This base-coat is a white lacquer that is applied in order to cover the outer metal surface of the can. The lacquer is fed from a lacquer reservoir to the lacquer application unit, in which the lacquer first is transferred to a rotating gravure roller. The gravure roller is abuttingly arranged against the periphery of a correspondingly rotating application roller to evenly transfer lacquer thereto. A can is fed to the application unit and is applied against the periphery of the application roller, so that a film of wet lacquer is transferred onto the surface area of the rotating can. It is also conceivable to arrange a similar lacquer application unit after the printer unit, in order to apply transparent lacquer, so-called over-varnish, to the graphical print on the can. In this case, the under-lying base-coat layer might be omitted. It is also conceivable to apply a transparent lacquer, so-called size-coat, directly to the metal surface of the can, which after drying is fed to the printer unit. In general, the graphical print on such a size-coat layer is not covered by over-varnish.
Traditionally, the lacquer application process is monitored by an operator taking out random samples of the lacquer-coated cans when these have been subsequently dried in the oven. Typically, two cans per shift for each apparatus are analyzed with respect to their dry lacquer film weight. Such an analysis is time-consuming and could last for up to 30 minutes. Evidently, errors in the production apparatus might not be discovered at all by this analysis, or at least not in due time. If excessive amounts of base-coat is applied to the cans, the subsequent graphical print will be unable to completely cover the thick white base-coat and the cans will assume a gray tint. Such gray cans must be discarded, leading to increased production costs. Excessive amounts of lacquer could also lead to unwanted deposition of lacquer on the inside of the cans. Since the can production apparatus for economical reasons cannot be shut down during the analysis, a large amount of defect cans will be produced in case of errors. Typically, the production rate in such an apparatus is 1,000-2,200 cans per minute. Since errors are not discovered in time, there is also a risk for failure in the equipment, leading to high cleaning/down-time costs for the application unit or the printer unit. Excessive amounts of lacquer could result in cans not being fully dried after the oven. Such cans must be discarded. Further, when cans are being processed in a subsequent necking unit, wrinkles might be formed in the neck area of the can if too much base-coat or over-varnish has been applied thereto. On the other hand, too small amounts of lacquer on the cans might lead to aluminum pick-up in subsequent equipment, e.g. the necking unit.
Obviously, there is a need for continuous monitoring of the amount of lacquer applied to each can, at least on an average basis.
In other technical fields, devices for consumption monitoring are previously known. An apparatus for application of adhesive on veneers is disclosed in Japanese patent publication No. 60-122320. In that apparatus, an intermediate storage tank is arranged between a reservoir for adhesive and a fluid dispenser. A non-contacting level gauge is provided at the tank to measure the height of the liquid level of adhesive in the tank. A counter is arranged to measure the number of adhesive coated veneers passed through said fluid dispenser while the liquid in the tank decreases between an upper and a lower level, as measured by the level gauge. A processing means is connected to the level gauge and the counter, so as to calculate the adhesive consumption per veneer.
A similar device is disclosed in German patent publication No. 39 25 016 (A), in which an apparatus for application of liquid glaze on ceramic tiles has an intermediate storage tank between a glaze reservoir and an application unit. A non-contacting level gauge is arranged to indicate an upper and a lower glaze level in the tank. During operation, the supply of glaze to the tank is interrupted and the rate at which the glaze drops from the upper to the lower level is measured and compared with a reference value for assessing the weight of glaze deposited on each article.
One drawback of the above arrangements is their inherent sensitivity to irregularities on the liquid surface in the intermediate storage tank. Often, the liquid must be continuously stirred in the tank. Such stirring will result in surface irregularities, typically in the order of 5-10 mm, creating errors when detecting the liquid at the upper or the lower levels in the tank.
These errors will be directly transferred into the calculation of the liquid consumption. The stirring could also cause foaming in the tank. By such foaming the liquid level in the tank is no longer unambiguously defined and might therefore be difficult to identify with a level gauge. Further, their sensitivity to surface variations make these arrangements less suitable for use in a can production apparatus for another reason. In a can production apparatus, a rejection sensor is arranged at the application unit to identify defect cans before they reach the application roller. When a defect can is detected and should be rejected, the whole application unit is retracted so that the defect can, and often the preceding and subsequent cans as well, passes the application roller without being coated with lacquer. A tank being connected between the application unit and the lacquer reservoir would inevitably be subjected to vibrations due to the retraction of the application unit, resulting in such measurement errors as described above.
Their sensitivity to surface variations is not the only problem of these known devices, but also that the operator is unable to distinguish accurately measured values from measured values that are influenced by surface irregularities etc.
Further, the consumption is only calculated intermittently, i.e. during time periods when the lacquer supply is shut off and the liquid level descends in the tank. Thus, fluid consumption is not monitored at all times during operation of the apparatus.
It should be noted that a can production apparatus operates at extremely high production rates, typically 1,000-2,200 cans per minute, the amount of lacquer applied to each can being small; approximately 200 mg wet base-coat per can, or approximately 150 mg wet over-varnish per can for a typical beverage can of aluminum or steel material. Deviations of more than about 15-30% are unacceptable for the reasons given by way of introduction. Thus, fluid consumption monitoring must be performed at high accuracy.
It is therefore an object of the present invention to provide a method and device for monitoring the consumption of a fluid in an application unit to obviate, or at least alleviate, the problems and limitations of the prior art systems.
It is a further object of the invention to to provide a method and device allowing an operator to identify unwanted trends in the fluid consumption, so that correcting measures can be taken at an early stage.
A yet further object of the invention is provide a method and device allowing an operator to identify incorrectly calculated consumption values.
Still another object of the invention is to provide a method and device for monitoring fluid consumption that can be installed at low cost, with only minor modification of existing production apparatus.
One further object of the invention is to provide a method and device for monitoring the consumption of fluid at all times during operation of the application unit.
Still another object of the present invention is to provide a method and device allowing the consumption of a fluid in an application unit to be monitored at high accuracy also when there is a need for continuous stirring of said fluid before it is fed to said application unit.
Another object of the invention is to provide a method and device allowing the consumption of a fluid in an application unit to be monitored at high accuracy also when said unit is subjected to vibrations.
According to the invention there is provided a method for monitoring the amount of fluid being applied to items in a fluid application unit, this unit being connected to a fluid reservoir via an intermediate storage tank, the tank having an inlet pipe connected to the reservoir and an outlet pipe connected to the application unit, and valve means being arranged in the inlet pipe for movement between a closed position and an open position. The method comprises the steps of: intermittently moving the valve means from the closed to the open position, thereby admitting fluid into the tank and raising a fluid surface therein from a lower level to an upper level; measuring an amount of fluid entering the tank by means of a flow meter arranged in connection with the inlet pipe; counting a number of items being applied with fluid in the fluid application unit; calculating a first value indicating the total amount of fluid passing the flow meter during a time period between two subsequent movements of the fluid surface to one and only one of said levels; calculating a second value indicating the total number of items being applied with fluid during said time period; and calculating a fluid consumption value per item by dividing the first value with the second value.
By this method, the amount of fluid being applied to each item is calculated as an average over said time period. By repeatedly calculating such average amounts and successively presenting them to an operator, undesired trends can be distinguished at an early stage.
The use of a flow meter allows the actual amount of fluid entering the tank to be measured at high accuracy.
Further, the consumption of fluid per item is monitored also during a filling sequence, i.e. when fluid is admitted into the tank. It is also possible to assess the accuracy of calculated fluid consumption value per item by examining the difference in the amount of fluid entering the tank from one filling sequence to the next. This difference indicates the magnitude of the surface irregularities in the tank during the measurement. Further, the method of the invention can be incorporated in present application units at low cost, with only minor modification of existing equipment. All, or at least a major part of the steps of the inventive method can be controlled by a computer program executed on a conventional computer.
According to a preferred embodiment, the inventive method also comprises the steps of sensing the position of said valve means, and defining the upper and lower levels based on a movement of the valve means to the closed and open positions, respectively. In this embodiment, both the flow of fluid into the tank and the number of coated items are calculated during a time period defined by two subsequent movements of the valve means to one and only one of its positions. This has proved to reduce the sensitivity to irregularities on the fluid surface compared to prior art devices.
According to a first aspect of the invention, the one and only one level is the lower level, and the valve means is controlled to open and close based on level signals from a level sensor associated with the tank. According to a second aspect of the invention, the one and only one level is the upper level, and the valve means is controlled to open and close based on level signals from a level sensor associated with the tank.
According to a third aspect of the invention, the one and only one level is the lower level, and the valve means is controlled to open based on a level signal from a level sensor associated with the tank and close when a predetermined amount of fluid has passed the flow meter. In the third aspect, there is no need for a level sensor to detect the upper level in the tank.
In one preferred embodiment of the invention, the calculated fluid consumption value per item is sent to a display means for graphical presentation of a succession of such values. This allows an operator to identify undesired trends in the fluid consumption, so that correcting measures can be taken at an early stage.
According to a further aspect of the invention there is provided a device for carrying out the method.